ABSTRACT Myelodysplastic syndromes (MDS) comprise a group of clonal hematologic malignancies characterized by ineffective hematopoiesis coupled with morphologic dysplasia, and generally remains incurable by existing non- transplant therapy. The Ten-eleven translocation (TET) proteins constitute a family of enzymes that assure proper regulation of gene expression through DNA demethylation, and TET2 is one of the most frequently mutated genes in MDS. Tet2?s roles in hematopoiesis have largely been studied in knockout mice; Tet2 deletion causes progressive defects in hematopoiesis, including the aberrant self-renewal of hematopoietic stem cells (HSCs), and Tet2-deficient mice develop a chronic myelomonocytic leukemia-like disease. However, as these mice lack the entire protein, it is not known how the non-catalytic functions of Tet2 contribute to HSC function and MDS pathogenesis. In order to delineate the catalytic and non-catalytic functions of Tet2, we have established Tet2 catalytically inactive mice. Using this new genetic mouse model, we have obtained the compelling evidence that Tet2 catalytic mutant mice have less pronounced hematologic phenotypes than Tet2 knockout mice. These preliminary findings led us to hypothesize that, in addition to its enzymatic roles in DNA hydroxylation/demethylation, Tet2 is critical for the proper governance of HSCs and suppression of MDS in a catalytic-independent manner. In this study, we will conduct a comparative epigenomic, transcriptomic and hematologic analyses of Tet2 catalytic mutant and knockout mice to establish the non-catalytic requirements of Tet2 in HSC homeostasis and MDS suppression. We have three main goals: (1) to determine the contributions of non-catalytic Tet2 functions to HSC self-renewal and differentiation, (2) to define the suppressive roles of non- catalytic functions of Tet2 in loss-of-Tet2-driven MDS, and (3) to identify non-catalytic targets of Tet2 responsible for suppressing aberrant HSC homeostasis and MDS development. The proposed research will have a major impact on the MDS field by defining the contribution of the non-catalytic functions of TET2 to MDS etiology, and can lead to new therapeutic approaches for the treatment and management of MDS and other hematologic disorders.